Tetrabromobisphenol-A is one of the most widely used brominated flame retardants in the world. It is used extensively to provide flame retardancy for styrenic thermoplastics and for some thermoset resins.
Processes used for producing tetrabromobisphenol-A generally fall into three categories. The first category includes processes in which substantial amounts of methyl bromide are co-produced along with the tetrabromobisphenol-A. Generally, up to 40-50 pounds of methyl bromide can be expected per 100 pounds of tetrabromobisphenol-A produced. In most cases, the processes within this first category feature reacting bisphenol-A and bromine in methanol. The ring-bromination of the bisphenol-A is a substitution reaction which generates one mole of HBr per ring-bromination site. Thus, for the production of tetrabromobisphenol-A, four moles of HBr are generated per mole of tetrabromobisphenol-A produced. The HBr in turn reacts with the methanol solvent to produce the methyl bromide co-product. After the bisphenol-A and bromine feed are finished, the reactor contents are cooked for one to two hours to complete the reaction. At the end of the reaction, water is added to the reactor contents to precipitate out the desired tetrabromobisphenol-A product.
The second category of processes features the production of tetrabromobisphenol-A without the co-production of substantial amounts of methyl bromide and without the use of oxidants to convert the HBr to Br.sub.2. See for example U.S. Pat. Nos. 4,990,321; 5,008,469; 5,059,726; and 5,138,103. Generally, these processes brominate the bisphenol-A at a low temperature, e.g., 0 to 20.degree. C., in the presence of a methanol solvent and a specified amount of water. The water and low temperature attenuate the production of methyl bromide by slowing the reaction between methanol and HBr. The amount of water used, however, is not so large as to cause precipitation of the tetrabromobisphenol-A from the reaction mass during the bromination reaction. Additional water for that purpose is added at the end of the reaction. This type of process typically uses a fairly long aging or cook period after the reactants have all been fed, and requires additional process time for the final precipitation of tetrabromobisphenol-A via the last water addition.
In the third category are those processes which feature the bromination of bisphenol-A with bromine in the presence of a solvent and, optionally, an oxidant, e.g., H.sub.2 O.sub.2, Cl.sub.2, etc. See for example U.S. Pat. Nos. 3,929,907; 4,180,684; 5,068,463 and Japanese 77/034620 B4 77/09/05. The solvent is generally a water-immiscible halogenated organic compound. Water is used in the reaction mass to provide a two-phase system. As the bisphenol-A is brominated, the tetrabromobisphenol-A is formed in the solvent. The co-produced HBr is present in the water. When used, the oxidant oxidizes the HBr to Br.sub.2, which in turn is then available to brominate more bisphenol-A and its under-brominated species. By oxidizing the HBr to Br.sub.2, only about two moles of Br.sub.2 feed are needed per mole of bisphenol-A fed to the reactor. To recover the tetrabromobisphenol-A from the solvent, the solution is cooled until tetrabromobisphenol-A precipitation occurs. The cooling of the solution to recover tetrabromobisphenol-A entails additional expense and process time.
Highly effective process technology for producing tetrabromobisphenol-A is described in commonly-owned U.S. Pat. Nos. 5,527,971, 5,723,690, and 5,847,232, and in commonly-owned co-pending U.S. patent application Ser. Nos. 08/945,158, filed Apr. 18, 1996; 09/096,332, filed Jun. 11, 1998; 09/149,225, filed Sep. 8, 1998; 09/288,195, filed Apr. 8, 1999; and 09/329,374, filed Jun. 10, 1999. One of the factors involved in achieving the highly desirable results made possible by these commonly-owned processes is maintenance of excess unreacted bromine in a liquid phase of the reaction mass during the bromination. The liquid phase in turn is one in which tetrabromobisphenol-A is relatively insoluble under the reaction conditions used. Thus tetrabromobisphenol-A precipitates in high yield essentially as soon as it is formed, and this high-yield precipitate formation occurs throughout substantially the entire period that bromination is occurring.